Height and width are provided in a television picture by means of timebase circuits which generate ramp currents, in order to drive the electron beam of the tube, either vertically down or horizontally across the tube. The simplest form of known timebase circuit generates a linear sawtooth waveform.
A problem occurs in known circuits due to variations in tube EHT voltage. When a picture becomes brighter the EHT voltage drops due to extra loading.
The drop in the EHT voltage causes the picture height to lengthen and the width to increase due to extra deflection of the electron beam since the electrons accelerated by a reduced EHT voltage have reduced energy when passing through the deflection coils.
This invention seeks to provide a timebase circuit in which the above mentioned problem is mitigated.